This invention concerns a display device for displaying video data (including image data and text data); and, more particularly, the invention relates, for example, to a liquid crystal display device, CRT (Cathode Ray Tube) display device, plasma display device or EL (Electroluminescence) display device.
Existent color conversion methods for converting video data received from video signal generation devices, and the conversion devices therefor, have been adapted, as disclosed, for example, in J-P-A No. 11-275375, to set correction values to lattice point data for strengthening positions in a possible range of color values after conversion to a poly-dimension lookup table so as to allow values out of the possible range for the color values after conversion, to thereby conduct desired color conversion as much as possible, input color signals for color conversion to an address generation section upon color conversion and conduct interpolation by an interpolation operation section based on lattice point data outputted from the poly-dimension lookup table to obtain color values after conversion corresponding to the inputted color signals, and, when the color values after conversion are out of the possible range, convert them to boundary values by a gradation conversion section.
However, although the technology described above involves a basic concept of setting lattice point color values for the boundary portions of a possible range for the color values after conversion to the poly-dimension lookup table and conducting an operation for the portion between lattice points, thereby enabling color conversion without increasing the capacity of the lookup table, it does not mention a lookup table setting means for analyzing input video data to produce an optimal display corresponding to the condition of the input video data.
Further, although the technology described above involves a color conversion method for video data based on the setting of a lookup table and an operation method for the lattice points based on a set value, it does not describe means for obtaining a satisfactory display, for example, by combining back light control.
As another technique, it has been known to expand a bright gradation or dark gradation by a γ-correction circuit corresponding to a high or low level of APL (Average Picture Level) of video signals to improve the contrast of display images in a liquid crystal display device. For example, a γ-correction circuit described in J-P-A No. 6-6820 has a γ-correction memory that stores γ-correction data for white level expansion and a γ-correction memory that stores γ-correction data for black level expansion and selects one of the γ-correction memories depending on whether the APL of the video signals is higher or lower than a predetermined value, to conduct γ-correction for the video signal. This can expand the bright gradation or dark gradation in accordance with a high or low level of APL in the video signal and can improve the contrast of the displayed images in the liquid crystal display device.
Video signals displayed on the liquid crystal display devices includes video images of television broadcasting, as well as video signals regenerated from VTR or DVD, video images photographed by video cameras and video images prepared by computer graphics. Further, since the number of broadcasting channels has been increased greatly by transfer from existent analog broadcasting to digital broadcasting, such as satellite broadcasting, video signal sources have become more and more versatile. Further, such versatile video signals have been introduced also in computers in addition to existent television broadcasting, where they are displayed as display data of computers, and, further, such video signals will be processed and fabricated and displayed on display devices.
In a case of displaying such versatile video signals on liquid crystal display devices, various kinds of γ-correction memories have to be provided corresponding to gradation characteristics for all sorts of video signals in the prior art, which is adapted to select one of plural γ-correction memories previously provided in accordance with a high or low APL in the video signals to conduct γ-correction for the image signals. Further, video image scenes actually change sequentially and currently with time for video signals, but provision of a number of γ-correction memories optimal to each of the video image scenes requires a great amount of memory capacity resulting in an increase of the cost, and so it is not practical. Further, in the selection of the γ-correction memory in accordance with the APL in the video signals, one identical γ-correction memory is selected for different video image scenes so long as the APL is i dentical. However, in the case of a low APL, for instance, this means that an identical γ-correction is applied irrespective of the fact that the entire screen shows a dark video image scene on average, or a video image scene in which a bright area is locally present in entirely a dark area. While γ-correction should be different between such cases, the γ-memory is selected in accordance with the APL in the video signals in the prior art, so that fine γ-correction depending on the video image scenes can not be conducted.
Furthermore, in CRTs used generally so far as display devices, electric signal and brightness are in a n2.2 but the liquid crystal display device has a characteristic such that the relation between the amount of light transmitting the liquid crystal and the electric signal is saturated both in a dark a rea and a bright area, as shown in FIG. 25. Thus, it is necessary to conduct γ-correction for a video signal while taking such a characteristic, which is inherent to the liquid crystal display device, into consideration.